My French is pretty rusty, but I saw in one report
Justice considers that the lack of moderation, cooperation with law enforcement and the tools offered by Telegram (disposable number, crypto, etc.) makes him an accomplice in drug trafficking, pedocriminal offences and fraud
It’s because (according to a translated French article I read and some other commentary), the app has encryption and privacy settings but the average criminal user isn’t using them so their criminal activity is blatantly obvious on the app and Telegram the company has refused to moderate that activity or those accounts.
While I don’t think that means he should be held solely responsible I do understand why a country might take exception to such a company openly allowing criminal activity with no effort whatsoever at moderation. Someone should be held accountable for that.